1. Field of the Invention
This invention generally relates to the art of lithographic printing. More specifically, it relates to a lithographic printing plate that can be made without involving developing processing, a method of making the printing plate, a lithographic printing method in which platemaking can be achieved on a printing press, and a printing apparatus enabling such platemaking.
2. Description of the Related Art
In the field of printing, emergence of a PS plate which is easy to make with rapidness has made lithographic printing supplant intaglio printing and letterpress printing. With the recent broadening of the printing market and the ever increasing demand for cost reduction, nevertheless, there has now been demands for development of new technologies which will bring more ease and rapidness in platemaking and reduction of industrial waste such as a waste developer.
To meet the market demands, various techniques have been proposed, in which a printing plate precursor having a photocatalyst compound and capable of changing its polarity between hydrophilicity and hydrophobicity on being irradiated with active light is used to make a printing plate of which the surface comprises hydrophilic areas and hydrophobic areas. These techniques are seen as extremely convenient because a printing plate precursor undergoes a polarity change simply upon being irradiated to form a printing plate having ink-receptive areas and ink-repellent areas according to whether irradiated or non-irradiated without. No developing processing is needed. Accordingly, the focus of the latest studies has been directed on how to achieve high print quality or to lengthen the press life of printing plates.
For example, JP-A-11-105234, JP-A-11-138970, JP-A-11-138971, JPA-11-143055, and JP-A-11-174664 disclose methods of platemaking and printing, in which a printing plate precursor having a photocatalytic metal compound thin film on its surface is imagewise exposed to active light to form a hydrophilicity/hydrophobicity distribution, and the printing plate after use can be reused by removing residual ink.
The printing method utilizing the character of a photocatalyst charging its polarity on irradiation for making an imagewise change in ink receptivity has a great merit of convenience. It has now been demanded to improve printing quality and press life in addition to this merit. To meet the demand, improvement on the contrast between irradiated areas and non-irradiated areas has been desired. The above-recited publications teach that photocatalyst titanium dioxide displays an appreciable change in polarity between hydrophilicity and hydrophobicity and is therefore highly effective for the contrast.
Digitization technology has been widespread in computer-aided processing, storing and outputting image information, and coupling the digitized image output with a platemaking process has been studied for rationalization of platemaking and printing processes. In particular, computer-to-plate (CTP) technology has been attracting attention, in which a plate precursor is directly scanned with light according to computer-processed image information to provide a printing plate without using a lith film. It is an important technical subject to develop a plate precursor, a printing method and a printing apparatus fit for CTP printing.
Use of photocatalytic titanium dioxide enables a CTP printing system using a rewritable printing plate precursor and will be a means meeting the market demand. In the practice, however, a photocatalytic film of titanium dioxide has low sensitivity and requires irradiation energy of several hundreds of millijoules to several joules per cm2 for making a polarity change. That is, a printing plate precursor having a titanium dioxide film takes a lot of time for exposure, which ruins platemaking workability. A powerful ultraviolet laser having an output power of 10 W or higher might enable short time exposure but is too expensive to be applied to a direct printing press in the practice.
An object of the present invention is to provide a printing method in which a printing plate can be made conveniently with high sensitivity and presents excellent printing quality and a long press life and an apparatus for carrying out the method. Specifically, the object is to provide a printing method in which a printing plate can be prepared by direct imagewise exposure without involving developing processing, suffers from little background staining, and has a long press life.
Another object of the present invention is to provide a printing apparatus having the above-described platemaking system therein.
Still another object of the present invention is to provide a printing method and a printing apparatus which enable repeated use of a printing plate precursor.
In order to accomplish the above objects, it is required that a photocatalyst titanium dioxide film be capable of forming an image in a practically short exposure time. The present inventors have conducted extensive researches into a means for increasing the polarity conversion efficiency of titanium dioxide, paying their attention particularly to the spectral sensitivity of titanium dioxide. As a result, they have found that the objects are achieved by:
A lithographic printing method comprising forming a uniform layer of a hydrophobic substance on the entire surface of a printing plate precursor having a titanium oxide-containing surface layer, imagewise irradiating the surface of the precursor with active light to form hydrophilic areas to make a printing plate, wherein the active light is far-ultraviolet light having a wavelength of 250 to 320 nm.
The lithographic printing method of the invention includes the following preferred embodiments:
The far-ultraviolet light is emitted from a solid state laser having an oscillation wavelength of 256 nm or a low-pressure mercury lamp having a vapor pressure of 0.1 kPa or lower.
The layer of the hydrophobic substance is formed to such a thickness as to have a contact angle with a water drop of 70 to 120xc2x0.
The printing plate precursor comprises a grained aluminum support having provided thereon the titanium oxide-containing surface layer.
The hydrophobic substance is a subliming solid or a volatile liquid, and the layer of the hydrophobic substance is formed by condensing vapor of the hydrophobic substance on the titanium oxide-containing surface layer.
The hydrophobic substance is an organic polymer, and the layer of the hydrophobic substance is formed by spraying a solution or dispersion of the hydrophobic substance onto the titanium oxide-containing surface layer.
The layer of the hydrophobic substance is formed on the titanium oxide-containing surface layer by spread coating, spray coating, vapor condensation, gas contact or dip coating.
The printing plate after use is cleaned to remove residual ink and reused as a printing plate precursor.
The objects of the present invention is also accomplished by:
A lithographic printing apparatus having (1) a part in which a printing plate precursor having a titanium oxide-containing surface layer is fixed, (2) a part in which a layer of a hydrophobic substance is formed on the entire surface of the printing plate precursor, (3) a part in which the printing plate precursor with the hydrophobic layer is imagewise irradiated with far-ultraviolet light having a wavelength of 250 to 320 nm to form a printing plate having an imagewise hydrophilic areas/hydrophobic areas distribution, (4) a part in which ink is fed to the hydrophobic areas, and a fountain solution is fed to the hydrophilic areas, and (5) a part in which the printing plate having ink on the hydrophobic areas thereof and the fountain solution on the hydrophilic areas thereof is brought into contact with a printing substrate to carry out printing.
The lithographic printing apparatus according to the invention includes the following preferred embodiments:
The parts (1) to (5) are disposed around a plate cylinder.
The apparatus further has (6) a part in which the printing plate after use is cleansed with an ink solvent to make it reusable as a printing plate precursor.
The concept of the present invention is essentially based on utilization of far-ultraviolet light to which titanium dioxide exhibits high photo-sensitivity in showing polarity conversion (hereinafter simply referred to as photo-sensitivity or sensitivity). This basic feature is explained with reference to FIG. 1 showing a spectral sensitivity characteristic curve of titanium dioxide, in which a requisite quantity of energy for polarity conversion is plotted against wavelength. As shown in FIG. 1, the sensitivity of titanium dioxide to wavelengths of 350 nm or shorter is several times as much as that to the light in the near-ultraviolet region. Therefore, utilization of light in the far-ultraviolet region in imagewise exposure will make it feasible to increase the sensitivity even higher than 100 mJ/cm2. This brings appreciable reduction in exposure time, leading to accomplishment of the objects of the present invention. The inventors have found that the concept can be materialized by using a low-pressure mercury lamp or a solid state laser as a light source.
On the other hand, the sensitivity of a printing plate precursor having a photocatalyst film varies depending on the material and the thickness of a layer of a hydrophobic substance (hereinafter referred to as a hydrophobilizing film) provided on the photocatalyst film. Accordingly, the objects of the invention are not always achieved merely by specifying the irradiation wavelength. In general, the thinner the hydrophobilizing film, the higher the sensitivity. However, as the hydrophobilizing film is made thinner, the hydrophobilizing effect decreases, and so does the press life. To secure sufficient ink receptivity for making a distinguishable contrast on a hydrophilic background, the hydrophobilizing, film should have a water contact angle of at least 60xc2x0 and, for practicability, 70 to 120xc2x0. The hydrophobilizing film is required to have such a thickness that the above range of a water contact angle is maintained on non-irradiated areas after imagewise exposure to far-ultraviolet light. It is not until the material and thickness of the hydrophobilizing film are properly selected to satisfy this requirement that both the photo-sensitivity and the hydrophobicity of non-irradiated areas can be secured in the system using far-ultraviolet light for imagewise exposure. In other words, only a combination of the irradiation wavelength range of 300 nm or shorter and the selected thickness of the hydrophobilizing film makes it possible to obtain a sensitivity of 100 mJ/cm2 or higher through the hydrophobilizing film and a distinct hydrophilicity/hydrophobicity contrast between irradiated areas and non-irradiated areas.
A low-pressure mercury lamp and a solid state laser used in the invention are inexpensive, small enough to be incorporated into a printing press, and capable of forming an image within a practical exposure time. Use of these light sources realizes a direct printing system capable of making a printing plate in situ and meeting the market demand for CTP printing.
The lithographic printing method of the invention comprises forming a layer of a hydrophobic substance on a plate precursor having a titanium dioxide-containing surface layer to hydrophobilize the overall surface of the precursor, imagewise exposing the precursor with active light in the far-ultraviolet region to selectively form hydrophilic areas. There is thus obtained a printing plate having a distribution of irradiated areas (the areas having lost the hydrophobicity) and non-irradiated areas which retain hydrophobicity. According to the printing system of the invention constituted of the printing method, the printing plate precursor, and the printing apparatus capable of platemaking, the platemaking step is simple and convenient, no developing processing is involved, image formation is achieved in a short time, and practical printing quality and contrast are secured.